In the exploration of oil, gas and geothermal energy, drilling operations are used to create boreholes, or wells, in the earth. Conventional drilling involves having a drill bit on the bottom of the well. A bottom-hole assembly is located immediately above the drill bit where directional sensors and communications equipment, batteries, mud motors, and stabilizing equipment are provided to help guide the drill bit to the desired subterranean target.
A set of drill collars are located above the bottom-hole assembly to provide a non-collapsible source of weight to help the drill bit crush the formation. Heavy weight drill pipe is located immediately above the drill collars for safety. The remainder of the drill string is mostly drill pipe, designed to operate under tension. A conventional drill pipe section is about 30 feet long, but lengths vary based on style. It is common to store lengths of drill pipe in “doubles” (2 connected lengths) or “triples” (3 connected lengths). When the drill string (drill pipe, drill collars and other components) are removed from the wellbore to change-out the worn drill bit, the drill pipe and drill collars are set back in doubles or triples until the drill bit is retrieved and exchanged. This process of pulling everything out of the hole and running it all back in is known as “tripping.”
Tripping is non-drilling time and, therefore, an expense. Efforts have long been made to devise ways to avoid it or at least speed it up. Running triples is faster than running doubles because it reduces the number of threaded connections to be disconnected and then reconnected. Triples are longer and therefore more difficult to handle due to their length and weight and the natural waveforms that occur when moving them around. Manually handling moving pipe can be dangerous.
It is desirable to have a drilling rig with the capability to reduce the trip time. One option is to operate a pair of opposing masts, each equipped with a fully operational top drive that sequentially swings over the wellbore. In this manner, tripping can be nearly continuous, pausing only to spin connections together or apart. Problems with this drilling rig configuration include at least costs of equipment, operation and transportation.
Tripping is a notoriously dangerous activity. Conventional drilling practice requires locating a derrickman high up on the racking module platform, where he is at risk of a serious fall and other injuries common to manually manipulating the heavy pipe stands when racking and unracking the pipe stands when tripping. Personnel on the drill floor are also at risk, trying to manage the vibrating tail of the pipe stand, often covered in mud and grease of a slippery drill floor in inclement weather. In addition, the faster desired trip rates increase risks.
It is desirable to have a drilling rig with the capability to reduce trip time and connection time. It is also desirable to have a system that includes redundancies, such that if a component of the system fails or requires servicing, the task performed by that component can be taken-up by another component on the drilling rig. It is also desirable to have a drilling rig that has these features and remains highly transportable between drilling locations.